1. Field of the Invention
This invention relates to an improved process for producing o-alkylated phenols.
2. Description of the Prior Art
The synthesis of 2,6-dimethylphenol (2,6-xylenol), among other o-alkylated phenols, has heretofore been the subject of many studies, because it is useful as a raw material for the manufacturing of polyphenylene ethers having a wide range of utilities in the fields of synthetic resins.
Currently, a process for the ortho-alkylation of phenols is in industrial use. It involves the vapor phase reaction of a phenol with an alcohol in the presence of a solid acid catalyst such as alumina (refer to U.S. Pat. No. 2,713,037). However, in this process, the selectivity for the ortho-alkylation is insufficient. That is, the meta and para positions of the phenol as well as the ortho positions thereof are alkylated to a considerable extent, so that a complicated procedure for the separation and purification of the ortho-alkylated product is required.
Another industrial process is based on the use of a magnesium oxide catalyst (refer to U.S. Pat. Nos. 2,448,942 and 3,446,856). But, this catalyst has inherently low activity, so it requires reaction temperatures higher than 475.degree. C., practically higher than 500.degree. C., to perform the reaction sufficiently. Moreover, the life of the catalyst is not long enough, so the regeneration procedure must be required in a relatively short period of time for practical use.
In order to solve these problems, there have been proposed many kinds of catalysts, for example, those comprising various combinations of magnesium oxide and other oxides (refer to U.S. Pat. No. 4,097,411), those comprising various combinations of iron oxide and other components (refer to U.S. Pat. No. 3,953,529).
These catalysts, however, still have similar disadvantages mentioned above, partly they are insufficient in selectivity for ortho-alkylation and in service life and partly they induce the alkylation of undesired positions (other than the ortho positions) and the formation of polyalkylated products under such reaction conditions as to produce a satisfactorily high catalytic activity.
Meanwhile, a process for the selective ortho-alkylation of a phenolic compound having at least one ortho-positioned hydrogen atom by using a manganese oxide catalyst is disclosed in U.S. Pat. No. 3,971,832. The invention disclosed therein relates to a process for the ortho-methylation of a phenol which comprises contacting the vaporized phenol with an alcohol at a temperature of from 250.degree. C. to 500.degree. C. in the presence of a catalyst consisting essentially of trimanganese tetroxide previously calcined at a temperature of from 950.degree. C. to 1,500.degree. C. It is stated that one feature of the invention is to provide a process for the ortho-methylation of phenols with a very high degree of selectivity under mild reaction conditions. However, this process cannot succeed to provide sufficiently high degrees of conversion of the phenol and selectivity for o-cresol or 2,6-xylenol.
The present inventors made extensive studies on a catalyst system comprising manganese oxide as the major component, and have proposed a catalyst which is effective on a selective ortho-alkylation of phenols (refer to U.S. Pat. No. 4,227,023). However, in order to produce o-alkylated phenols advantageously on an industrial scale, it is necessary to reduce the accumulation rate of the hydrocarbon deposition on the surface of the catalyst, to further prolong the regeneration cycle of the catalyst, and consequently to prolong the catalyst life.